1. Field
This invention relates to the field of ceramics and particularly to dense, polycrystalline tungsten carbide (WC) bodies.
2. Description of the Prior Art
In the early 19OO's WC was densified by heating this oeramic to temperatures near 2000.degree. C., but the WC was far too brittle to make an acceptable cutting tool. Schroter initiated work in 1914 to make cemented carbides, which are composites which combine an abrasive ceramic (carbide phase) with a ductile metal (i.e., Co, Ni, Fe, etc.) by sintering the powder composite at temperatures above the melting point of the metallic phase. These ceramic-metal composites are also called by the acronym "cermets" and were designed to combine the abrasive properties of the carbide phase with the ductility of the metallic component. Cemented carbides, particularly WC-Co cermets, have been used extensively in metal cutting applications since 1927.
In order to increase the cutting speed, and hence the cutting efficiency, cutting tools of essentially pure ceramics (ceramics are inorganic, nonmetallic materials; particularly oxides, carbides, nitrides, borides, etc.) without metal additives, or with minor metallic additives introduced during the milling process, have been used since the introduction of Al.sub.2 O.sub.3 in the 1950's. The cutting speed has continually increased as Al.sub.2 O.sub.3 -TiC, SiAlON, Si.sub.3 N.sub.4, and SiC whisker-reinforced Al.sub.2 O.sub.3 ceramic cutting tools have been introduced into the market place. Cutting speeds as high as 25 surface meters per second (m/s) have been reported, and speeds between 5 and 15 m/s are routinely achieved with such essentially pure ceramics (i.e., low metallic impurities), whereas even ceramic-coated cemented carbides are generally limited to speeds below 7 m/s. The deformation of the metallic phase in the cermets at temperatures as low as 600.degree. C. generally limits the cutting speed for the cemented carbides.
In general, an acceptable cutting tool has room temperature toughness greater than 4 MPa-m.sup.1/2, flexural strength greater than 400 MPa, hardness greater than 15 GPa, and thermal conductivity greater than 20 W/mK. The above properties are minimum values and higher values are desirable. High toughness, strength, and thermal conductivity lead to tools with improved resistance to chipping, and thereby enable one to make interrupted cutting when metal machining. Cemented carbides used in drilling applications have toughness values in excess of 8 MPa-m.sup.1/2. Higher hardness correlates with improved wear resistance, although chemical compatibility is essential for good performance. Since temperatures in metal cutting applications reach 1000.degree.-1200.degree. C. at the tool-work piece interface, it is important to retain hardness and toughness at high temperatures. The greater efficiency of many ceramics, as compared to cemented carbides (i.e., cermets), is that they maintain wear resistance (i.e., hardness) and strength at elevated temperatures.
The toughness of ceramics are generally below that of cemented carbides, and those ceramics with toughness values approaching 8 MPa-m.sup.1/2 (i.e., Si.sub.3 N.sub.4 and SiC whisker-reinforced Al2O.sub.3) have poor thermal conductivity as compared to cemented carbides. Since high toughness and thermal conductivity are required to perform well in interrupted cutting or drilling applications, it would be desirable to find ceramics with improved toughness and thermal conductivity.
Despite the improved performance in metal machining operations due to the advent of ceramic-coated cemented carbides and ceramics, there are still applications where machining is still very slow. An excellent example is the machining of titanium alloys. Machining is normally accomplished with cemented tungsten carbide tools (1-5 micrometer WC grain size) with five to six weight percent cobalt made by conventional liquid phase sintering at temperature near 1400.degree. C. in vacuum or hydrogen. Maohining speeds for such tools are between 1.0 and 1.5 surface meters per second when machining titanium with depths of cut between 0.20 and 0.26 millimeters per revolution. This is very slow compared to machining of other metals and represents a barrier that many tool developers have sought to overcome. Numerous cemented carbides, ceramic-coated cemented carbides, and available ceramics have all been tested without success in order to find a cutting tool that can machine titanium alloys at higher speeds than WC-Co tools.
There is widespread disagreement on the thermal conductivity of essentially pure WC, with values ranging between 30 W/mK (as reported in Transition Metal Carbides and Nitrides by L. E. Toth (Academic Press, London, 1971)) to 121 W/mK (as reported by Kieffer and Benesovsky in Encyclopedia of Chemical Technology, 3rd Editicn 490-505 (1978)). When Batelle Columbus Labs published their Engineering Procerty Data on Selected Ceramics Vol. 2, Carbides in 1979 they stated that "limited and questionable thermal conductivity data of WC were not considered worthy of inclusion".
Data on the fracture toughness of WC in the absence of Co are sparse in spite of extensive data for cemented carbides. It is also not recognized that the fracture toughness of tungsten carbide, in the absence of metallic binding phases, is suitable for cutting applications. Chermant, et al. (Fracture Mechanics of Ceramics, Vol 4 edited by R. C. Bradt, D. P. H. Hasselman, and F. F. Lange, Plenum Press, New York, 891-901 (1978)) upon reporting a fracture toughness of 7.5 MPa-m.sup.1/2 for WC said that the value is very low.
It is well documented that WC has excellent hardness at temperatures to 1000.degree. C. as reported by Westbrook and Stover (High-Temperature Materials and Technology edited by I. E. Campbell and E. M. Sherwood, Wiley, New York, 312 (1967)). While the hardness of WC is approximately 18 GPa at room temperature, which is significantly lower than TiC (approximately 30 GPa), it is approximately 12 GPa at 1000.degree. C., which is much higher than TiC (approximately 4 GPa). The limitation of WC is that it oxidizes readily, but its utility in cutting tools would be possible due to its ability to transport heat away from the cutting tool tip assuming it has high thermal conductivity. Dense WC ceramics withhigh thermal conductivity, fracture toughness values approaching 8 MPa-m.sup.1/2, and high hot hardness are expected to find utility in a wide range of applications requiring wear resistance and high toughness at low temperatures (less than 300.degree. C.) for extended periods of time (hours), or where short excursions to high temperatures (less than 1200.degree. C.) are required.
Foster, et al. (J. Am. Ceram. Soc., 33 1, 27-33 (1950)) showed that it was possible to sinter WC with Co, Ni, CoO, or NiO additions as small as 0.25 wt. %. Due to their high sintering temperature (2000.degree.-2500.degree. C.) substantial grain growth occurred during sintering, resulting in low hardness due to large WC grains ( WC grains between 15 and 30 micrometers are evident in their microstructures). In addition, their long milling times (100-264 hours) in a WC-Co lined mill with cemented carbide media, certainly added excess liquid phase to promote sintering. Meredith and Milner (Pow. Met., 1, 38-45 (1976)) reported on the activated sintering of WC using Co additions up to 1 volume percent but failed to reach closed porosity. Other investigators (see Schwarzkopf, Refractory Hard Metals, MacMillan Co., New York 1953) have found that Ni or Fe additions of less than 1 volume percent promote sintering but no one has been able to densify WC without external pressure, even with additions of up to 1 volume percent metal, to closed porosity so that hot isostatic pressing is possible, and retain a microstructure which is suitable for commercial cutting and wear applications. It is also not recognized that tungsten carbide, in tne absence of impurities, densifies well by solid state sintering. Meredith and Milner found no densification of WC without Co additions during their investigation of enhanced sintering at temperatures up to 1400.degree. C.
In recognition of the interest in increasing the machining rate of titanium alloys and making ceramics with improved toughness and thermal conductivity, it would be a major innovation and improvement in the art to develop ceramics which readily machine titanium and can be used in other applications where high wear resistance and fracture toughness are required.